In a report published in the Jan. 26 issue of the journal Nature, biologist Peter Reich of the University of Minnesota and his colleagues found that the rate of plant metabolism, or respiration--and its related emissions of carbon dioxide--can be deduced from the nitrogen content of any plant.Plants carry out respiration during the dark hours when they, like animals, take in oxygen and give off carbon dioxide. During daylight, plants carry out photosynthesis, in which the process is reversed.Researchers have estimated that plant respiration releases five to 10 times as much carbon dioxide as fossil fuel burning. It's crucial, they say, to know the amount of plant emissions more accurately. Yet, "the amount of carbon dioxide given off by plants is one of the weak spots in models of global carbon cycling," Reich said.